Comment spam is the most annoying thing to web operators. Besides eating up bandwidth, comment spam can pollute web discussions area and which gives bad impression to visitors.
Apache HTTPD mod_security module can be configured to reduce web spam by filtering common keyword, content and referrer used by spam bots around the internet.
Here’s an example of .htaccess file to block common comment spam :
SecFilterSelective POST_PAYLOAD "(mortgage|viagra|poker|traffic|discount|medical|casino|lyrics|loan)"
Please ensure that your Apache installation has mod_security module enabled. The method is suitable to be used on websites that receive a lot of user comments like forums, blogs (including WordPress and Drupal) and photo gallery.
Note: This is not a full-proof solution as it depends on the use of keywords.